This invention relates generally to a carbon-segment commutator for an electric motor and a method for its manufacture.
Permanent magnet direct current motors are sometimes used for submerged fuel pump applications. These motors typically employ either face-type commutators or cylinder or xe2x80x9cbarrelxe2x80x9d-type commutators. Face-type commutators have planar, circular commutating surfaces disposed in a plane perpendicular to the axis of armature rotation. Barrel-type commutators have arcuate, cylindrical commutating surfaces disposed on the outer side surface of a cylinder that is positioned coaxially around the axis of armature rotation. Regardless of their commutating surface configurations, electric motors used in submerged fuel pump applications must be small and compact, have a long life, be able to operate in a corrosive environment, be economical to manufacture and operate and be essentially maintenance-free.
Submerged fuel pump motors must sometimes operate in a fluid fuel medium containing an oxygen compound, such as methyl alcohol and ethyl alcohol.
The alcohol increases the conductivity of the fuel and, therefore, the efficiency of an electrochemical reaction that deplates any copper motor components that are exposed to the fuel. For this reason, carbon and carbon compositions are sometimes used to form carbon segments with segmented commutating surfaces for the motors. This is because carbon commutators do not corrode or xe2x80x9cdeplatexe2x80x9d, as copper commutators do. Commutators with carbon segments also typically include metallic contact sections that are in electrical contact with the carbon segments and provide a terminal for physically connecting each electrical contact to an armature coil wire.
It is known to form a carbon commutator by first molding and heat treating a moldable carbon compound or machining heat-treated carbon or carbon/graphite stock. Such an arrangement is shown in German Disclosure 3150505.8. A commutator-insulating hub may be formed to support a metallic substrate. The hub may be molded directly to the metallic substrate. Slots are machined through the carbon article and the metallic substrate to separate the carbon article and substrate into a number of electrically isolated segments. An inner diameter, outer diameter and the commutating surface of the commutator may also need to be machined.
After the completed commutator is assembled to an armature, a clamshell mold may be positioned over the newly assembled commutator-armature in a final overmolding process. With face-type commutators, an open end of the clam shell mold is made to seal around commutator in a manner that leaves the commutating surface exposed. Insulator material is then injected into the clam shell mold. Once the insulator material has cured, the clam shell mold is removed. This final overmolding step protects copper armature windings and other corrosion-prone elements from chemically reacting with ambient fluids such as oxygenated fuels. The overmolding also secures wires to reduce potential for stress failures and to maintain a corrected dynamic balance level. Overmolding will also reduce windage losses in the pump.
When, in manufacturing a carbon commutator with a metallic substrate, cuts are machined into or through the metallic substrate, metal chips may be produced. These metal chips can lodge in the slots between carbon segments causing electrical failures. Machining into a metallic substrate can also expose the cut portions of the substrate to the corrosive effects of oxygenated fuels.
Where the carbon and metal substrate portions or a commutator are machined-through to form electrically isolated segments, some type of support structure must be provided to strengthen the commutator and mechanically bind the carbon segments and conductor sections together. Such support structures sometimes require substantial additional axial space for the commutator, which can increase the overall axial length of the armature-commutator assembly and or reduce the size and the quantity of wire wound in the armature.
For some types of electrical-conducting resin-bonded carbon compositions, an insulating surface skin characteristically forms on exterior surfaces of the composition as it cures. This skin forms an impediment to electrical contact between the carbon composition and the metallic conductor sections. Therefore, a carbon commutator using such a composition must provide an electrical path through the insulating surface skin.
One approach to solving these problems is disclosed in U.S. Pat. No. 5,386,167 issued Jan. 31, 1995 to Strobi (the Strobi patent). The Strobi patent shows a face-type commutator having eight carbon segments formed from an electrical-conducting resin-bonded carbon composition. To avoid problems associated with machining into metal substrates, the carbon segments are formed by overmolding a carbon disk onto eight pie-piece-shaped copper segments then radially cutting between the segments to form the electrically isolated carbon segments. A plastic substrate holds the copper segments in position for carbon overmolding and provides mechanical interlock between the carbon segments. However, the plastic substrate increases the axial thickness of the commutator. In addition, the Strobi patent does not provide structures that would provide an electrical path through carbon composition skinning or structures that might otherwise reduce electrical resistance.
U.S. Pat. No. 4,358,319 issued Nov. 9, 1982 to Yoshida et al. discloses a barrel-type carbon commutator assembly that includes an annular cylindrical array of carbon segments. Each carbon segment has an outer semi-circumferential side surface for making physical and electrical contact with a brush. A retention groove extends around an inner circumferential surface of the carbon segment array. The carbon segments are electrically isolated from each other by longitudinal cuts. A hub comprising insulating material is disposed within the annular carbon segment array and engages the retention groove at the top end of each carbon segment.
To manufacture this commutator Yoshida et al. discloses a method that includes the steps of forming an annular carbon cylinder with a retention groove, over-molding the carbon cylinder with insulator material to form a hub and machining slots in the over-molded barrel to form electrically isolated barrel segments. The electrical connections between carbon segments and coil wires are made by soldering or gluing the wires directly to the carbon segments themselves.
A fuel pump supplied by Bosch to Mercedes Benz shows a barrel-style commutator that includes a cylindrical commutating surface formed by a cylindrical array of carbon segments. Radial inner surfaces of the carbon segments form a composite inner circumferential surface of the carbon segment array. The carbon segments are electrically connected to respective coil wires by copper substrate sections soldered to the respective radial inner surfaces of the carbon segments. Each copper substrate section includes a terminal for supporting the end of a coil wire.
The Bosch commutator appears to be formed by fitting and soldering a tube portion of a copper substrate to the inner circumferential surface of the carbon cylinder. Radial cuts are then made to form and electrically isolate the carbon segments and copper substrate sections from each other. An over-molded insulator holds the carbon segments and copper substrate sections together. This process requires that a copper substrate be fabricated to include wire terminals and a tube portion closely toleranced to fit within the inner circumferential surface of the carbon cylinder. The Bosch process also requires that a difficult soldering operation be performed between the inner circumferential surface of the carbon cylinder and the outside diameter of the copper tube.
U.S. Pat. No. 5,255,426 issued Oct. 26, 1993 to Farago et al. discloses a face-type carbon commutator manufactured by first forming an annular or toroidal carbon cylinder comprising fine-grained electrical-grade carbon. Next, a cylinder base end surface is plated with a layer of a conductive material such as nickel. A layer of a conductive material such as copper is then plated over the nickel plating. The plated base end surface of the cylinder is then soldered to a substrate. Lateral slots are then machined axially downward into a top commutating surface opposite the base surface of the carbon cylinder. The slots are cut axially through the carbon and the copper substrate to form the electrically isolated carbon/copper commutator sectors.
What are needed are both face and barrel-type carbon-segment commutators that are stronger and provide lower electrical resistance through improved electrical contact between carbon segments and metallic substrates. Also needed are methods for manufacturing such commutators that are quick, easy and inexpensive.
In accordance with this invention a carbon-segment commutator assembly is provided in which a carbon disk is molded over a pre-stamped metallic substrate having an upturned projection, and an insulator hub is molded over the carbon-overmolded substrate prior to cutting radial slots. The commutator assembly comprises an annular array of at least two circumferentially-spaced conductor sections arranged around a rotational axis and an annular array or at least two circumferentially-spaced carbon segments formed of a conductive carbon composition. Each carbon segment is molded onto at least one surface of a corresponding one of the conductor sections with the annular array defining a segmented commutating surface of the commutator. An overmolded insulator hub is disposed around and between the carbon segments. The insulator hub mechanically interlocks the carbon segments. Each conductor section has at least one conductor projection that is at least partially embedded in a corresponding one of the overmolded carbon segments.
According to one aspect of the present invention, a method is provided for making a carbon-segment commutator assembly. The method includes providing the annular array of conductor sections then forming a carbon overmold by molding an electrical-conducting resin-bonded carbon composition onto the annular conductor section array. Inner grooves are formed in an inside surface of the carbon overmold opposite the commutating surface. Next, the insulator hub is formed by overmolding the carbon overmold and conductor section array with insulator material that at least partially occupies the inner grooves and mechanically interlocks the carbon segments. Finally, machining slots inward from the commutating surface of the carbon overmold to the inner grooves forms the annular array of electrically isolated carbon segments while electrically isolating the segments from each other.
Unlike prior art commutators, the filled inner grooves of the present invention leave only a thin section of the carbon segment to be machined through to electrically isolate the carbon segments. This provides at least three benefits: shallow slots result in a stronger and/or an axially shorter commutator, less machining time is required to cut the slots, and tool wear is reduced resulting in extended tool life.
In addition, the conductor projections of the present invention reduce electrical resistance by increasing surface area contact between the conductor sections and their corresponding carbon segments. The projections also provide lower electrical resistance through increased carbon to copper contact within the carbon segments and provide an electrical path through any insulating surface skin that might form over carbon segments made of certain carbon compositions.
In accordance with another aspect of the invention, the inner grooves are formed into the carbon composition as the electrical-conducting resin-bonded carbon composition is overmolded. This obviates the need to form the inner grooves in a separate step.
In accordance with another aspect of the invention, the annular array of carbon segments defines a segmented composite outer-circumferential commutating surface of the commutator. The overmolded insulator hub is disposed on an axial top end, base end and inner circumferential surfaces of the annular array of commutator sectors to mechanically interlock the commutator sectors.
In accordance with another aspect of the invention, a circular retention groove is disposed in the top end surface of the annular array of commutator sectors. A portion of the insulator hub is disposed within the retention groove to help bind the sectors together.
In accordance with another aspect of the invention each conductor section is at least partially imbedded in one of the carbon segments and includes a conductor tang that extends radially outward from that carbon segment.
In accordance with another aspect of the invention, radial interstices separate the carbon segments. Bach interstice has an inner groove portion filled with the hub insulator material and an unfilled outer slot portion. This construction electrically isolates the carbon segments while physically binding them together in an annular array.
In accordance with another aspect of the invention, the carbon segments comprise a composition of carbon powder and carrier material. The composition may comprise metal particles embedded in the composition of carbon powder and carrier material to improve electrical characteristics. The carrier material may be selected from the group consisting of phenolic resin, a thermoset resin and a thermoplastic resin. Graphite may account for 50-80% of the weight of the carbon composition.
In accordance with another aspect of the invention the inner grooves are formed as the electrical-conducting resin-bonded carbon composition is overmolded.
In accordance with another aspect of the invention a retention groove is formed in an axial top surface of the carbon overmold as the carbon overmold is formed. The insulator material is flowed over the cop surface and into the retention groove to further secure the segments after slotting. The outer circumferential surface is left exposed to serve as a commutating surface.
In accordance with another aspect of the invention, the carbon composition is molded both over and under the annular array of conductor sections. This embeds at least a portion of the conductor section array within the carbon composition.
In accordance with another aspect of the invention, a first metallic layer is plated onto an inner surface of each carbon segment. The metallic substrate sections are soldered to the respective plated inner surfaces of the carbon segments to provide strong mechanical and electrical connections between the carbon segments and their respective substrate sections. A second metallic layer may be plated over the first metallic layer. The first metallic layer may comprise nickel and the second metallic layer may comprise copper.
In accordance with another aspect of the invention, the metallic material of the first and/or the second metallic layer is deposited within pores disposed in the inner surface of each carbon segment to improve mechanical strength and electrical conductivity.
In accordance with another aspect of the invention, the solder connecting the carbon segments to the substrate sections includes an even distribution of flux. The flux is mixed with the solder paste before soldering to insure even flux distribution and improved mechanical and electrical contact.
In accordance with another aspect of the invention, the carbon segments each have a retention groove formed adjacent an axial top end of each respective carbon segment disposed opposite the inner surface. The hub is formed into the retention groove mechanically locking the carbon segments together.
In accordance with another aspect of the invention, each substrate section includes a tang extending integrally outward into the hub. The tang is embedded in the hub to form a stronger mechanical lock between the substrate sections and the hub.
In accordance with another aspect of the invention, the hub comprises a phenolic compound.
In accordance with another aspect of the invention, each carbon segment comprises a conductive carbon composition. The composition may include one or more materials selected from the group consisting of isostatic electrographite, carbon graphite, and fine-grained extruded graphite.
In accordance with another aspect of the invention, each metallic substrate section includes a terminal that extends radially outward from the hub. Each terminal may have a U-shape to facilitate attachment of coil wires.
In accordance with another aspect of the invention, a circular array of radial interstices separates the commutator sectors. According to one embodiment, each interstice has an inner groove portion filled with the hub insulator material and an unfilled outer slot portion.
In accordance with another aspect of the invention, a method is provided for constructing a carbon commutator in which an inner surface of an annular carbon cylinder is metallized. The inner surface is metallized by bonding a first layer of metallic material to the inner surface. A metallic substrate is then soldered to the metallized inner surface of the carbon cylinder. An annular insulator hub is then provided within the carbon cylinder and radial interstices are provided through the carbon cylinder and the metallic substrate to form the electrically isolated carbon/metal commutator sectors.
In accordance with another aspect of the invention, a second layer of metallic material is bonded to the inner surface of the carbon cylinder.
In accordance with another aspect of the invention, a layer of metallic material is electroplated to the inner surface of the carbon cylinder.
In accordance with another aspect of the invention, brush-type selective plating is used to electroplate the first layer of metallic material onto the carbon cylinder inner surface. Brush-type selective plating xe2x80x9cthrowsxe2x80x9d metal molecules/ions deeper into the carbon cylinder than conventional electrolysis techniques. This results in a stronger mechanical bond and a superior electrical connection. Brush-type selective plating is also used to electroplate the second layer of metallic material onto the carbon cylinder inner surface.
In accordance with another aspect of the invention, the inner surface of the carbon cylinder is metalized by forming a thin tin-based chemical reaction zone on the inner surface of the carbon cylinder that provides true molecular bonding resulting in superior mechanical strength and electrical conductivity. The chemical reaction zone is formed by providing a tin-based metallization layer including a chemical reaction zone at the inner surface of the carbon cylinder. This is done by forming a metallic powder mixture of tin with a transition metal such as chromium. A metallization paste is then formed by mixing the metallic powder mixture with an organic binder. The paste is applied to the base end surface by painting or stencil printing, and is fired to 800-900xc2x0 C. in an atmosphere including carbon monoxide. The paste may be fired in a nitrogen atmosphere because binder burnout will produce sufficient carbon monixide to support the reaction. In accordance with this same method, the substrate is soldered to the base end surface of the carbon cylinder by converting the metallization layer into a solder layer by reflowing a solder composition into the metallization layer.
In accordance with another aspect of the invention, the substrate is soldered to the carbon cylinder using a solder paste containing flux. This eliminates steps that would otherwise be required to properly distribute the flux. Solder may be applied to the inner surface of the carbon cylinder using a stencil printing process. Stencil printing reduces waste and contamination of other portions of the commutator structure. During the stencil printing process a stencil is placed over the inner surface or the carbon cylinder and a layer of solder paste is provided on the stencil and exposed portions of the carbon cylinder inner surface. The stencil is then removed from the carbon cylinder. This process leaves solder paste only in desired locations. After applying the solder paste, the substrate is aligned with the inner surface of the carbon cylinder and the substrate is then placed against the solder-coated inner surface of carbon cylinder. The assembly may then be placed in a reflow oven to help insure proper soldering.
In accordance with another aspect of the invention, a retention groove is provided in the top end of the cylinder before forming the hub. In addition, an inner groove portion of each radial interstice may be formed radially outward into the inner circumferential surface of the carbon cylinder before forming the hub instead of after.
In accordance with another aspect of the invention insulator, material is overmolded onto the carbon cylinder and metallic substrate in an insert molding process to form the hub. During the overmolding operation, the insulating material is allowed to flow into the retention groove. In embodiments with pre-formed inner grooves, the insulator material is also allowed to flow into the radial inner grooves.
In accordance with another aspect of the invention, in embodiments with pre-formed inner grooves, outer slot portions of the radial interstices are formed by machining the slot portions radially inward from an outer circumferential surface of the carbon cylinder. The outer slot portions cooperate with the insulator-filled inner groove portions to electrically isolate the commutator sectors.
In accordance with another aspect of the invention, the formation of the metallic substrate includes the stamping of a generally circular annular metallic substrate from a sheet of metal. The circular annular array of metallic substrate sections is stamped from the sheet of metal such that each substrate section includes a radially-outwardly-extending terminal and an inwardly extending tang. The substrate tangs are separated by radially-inwardly-extending slots. The substrate sections are connected by connector tabs that are easily machined through when the radial interstices are formed. Each terminal may be bent into a U-shape and a portion of each tang may be bent downward to improve mechanical retention in the overmolded hub material. The outwardly extending terminal may alternatively be stamped to form an insulation-displacement configuration.